The lower section, also known as the block, contains the pistons and crankshaft whereas the upper section also called the head holds the engine valves. The gasket which holds the sections together has holes in it which allows the engine coolant to circulate around the upper and lower sections as well as holes for oil to flow through. If the gasket blows, the engine oil will enter the coolant passages causing the engine to fail.
Moreover, damage to the cylinder connected to the engine block may cause the oil to seep into the radiator. The bolts that hold the cylinder head onto the engine block can sometimes experience wear and stretching which destroys the seal on the head gasket. Therefore, the oil and coolant will be forced to mix together after flowing into the gasket.
In addition, if the engine overheats then the cylinder head could get warped and cause the same problem. If a crack appears between the internal oil and coolant passages within the engine block then a problem will occur. The cracks can develop due to a strong impact such as a crash or by the engine freezing which can occur in bitterly cold conditions. Freezing can also occur if too much water is in the antifreeze or coolant mixture which could make it expand and crack.